One example of a conventional ink jet printer is shown in FIG. 1. The ink jet printer comprises a plurality of drop generators 2 for jetting ink drops 1 in the direction z according to a predetermined drop forming frequency. Charging electrodes 3 are provided for selectively charging the drops 1 according to printing signals as the ink drops 1 fly in the direction z. Deflecting electrodes 5 are inclined by an angle .theta. corresponding with the speed of movement of a recording sheet 4 which is moving in the direction y. Each deflecting electrode successively deflects, in a deflecting plane forming the same angle .theta., the flying ink drops 1a, 1b, . . . 1n. A sensor 6, including a light emitting unit 6a having a light emitting diode array and a light receiving unit 6b having a photo-transistor array, detects the positions of the flying ink drops 1a, 1b, . . . 1n in the direction x.
In an ink jet printer constructed in the described manner, the sensor 6 detects the positions in the direction x of the ink drops 1a, 1b, . . . 1n which pass through the sensor. The system is calibrated before the printing operation is started according to the displacement of the ink drops. In the printing operation, the ink drops 1 are jetted by each of the drop generators 2 forming a multi-nozzle. After being charged by the charging electrodes 3, the ink drops are deflected in a time-series manner in the deflecting plane having the angle .theta., to strike the recording sheet 4 in a selected pattern. Because of the displacement angle .theta., the ink drops 1a, 1b, . . . 1n form a straight line extended in the direction x. The ink drops which do not contribute to the printing are recovered by a gutter (not shown).
In the above described ink jet printer, the displacement in the direction x of the ink drops 1 is detected by the sensor 6 in which detecting light emitted from the light emitting unit 6a in the direction y is received by the light receiving unit 6b. Accordingly, there is a distance g in the direction y between the ink drop 1n, the last drop deflected, and the ink drop 1a deflected first from adjacent drop generators 2 owing to the angle .theta. (g=0.16 mm under certain conditions). However, the displacement in the direction y is not employed during calibration of the system, and, therefore, the print dots cannot be joined together with high accuracy. As a result, only limited improvement of the picture quality can be attained.
As indicated by the broken lines in FIG. 1, a light emitting unit 7a and a light receiving unit 7b may be disposed at both ends, in the direction x, of the sensor to detect the displacement.